Polarizing glass and preparation method thereof

ABSTRACT

A polarizing glass comprising geometrically anisotropic particles dispersed in an oriented manner in at least the surface of a glass base body. The glass base body is denoted by the weight percentages of 50-65 percent SiO 2 , 15-22 percent B 2 O 3 , 0-4 percent Al 2 O 3 , 2-8 percent ZrO 2 , 6 percent &lt;Al 2 O 3 +ZrO 2 &lt;12 percent, 6-16 percent R 2 O (where R denotes at least one from among Li, Na, and K), 0-3 percent Li 2 O, 0-9 percent Na 2 O, 4-16 percent K 2 O, Li 2 O+Na 2 O&lt;K 2 O, 0-7 percent BaO and/or SrO, and 0-3 percent TiO 2 . The glass base body comprises per 100 weight percent of essentially the above composition at least 0.15-1.0 percent Ag and at least the chemical equivalent to Ag of Cl and/or Br; and the geometrically anisotropic silver particles are metallic Ag particles. The polarizing glass is employed in optical products such as optical isolators.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to polarizing glasses employed inoptical products such as optical isolators, and more particularly, tohigh performance polarizing glasses comprising geometrically anisotropicmetallic silver particles. The present invention further relates tomethods of manufacturing these polarizing glasses.

[0003] 2. Description of Related Art

[0004] Polarizing glasses comprising geometrically anisotropic metallicsilver particles can be manufactured by the methods described inJapanese Patent Application Examined Publication No. Hei 2-40619(Referenced publication 1) and Japanese Patent Application Un-examinedPublication No. Sho 56-169140 (Referenced publication 2).

[0005] In these methods, glass comprising a silver halide isheat-treated to deposit out the silver halide and the glass is elongatedto lengthen the silver halide particles. The glass is then heat treatedin a reducing environment to reduce the silver halide particles tosilver, thereby manufacturing polarizing glass comprising geometricallyanisotropic silver particles.

[0006] For example, the following method is specifically described inReferenced publication 1.

[0007] A method of manufacturing glass articles exhibiting excellentpolarization in the infrared region of the spectrum from glassescontaining silver halide particles therein selected from the group ofAgCl, AgBr, and AgI, characterized by comprising steps in which (a) abatch for a glass containing silver and at least one halide selectedfrom the group of chloride, bromide, and iodide is melted and the meltshaped into a glass body of a desired geometry; (b) the glass body issubjected to a heat treatment at least above the strain point but not inexcess of 50° C. above the softening point of the glass for a period oftime adequate to cause the generation of AgCl and/or AgBr and/or AgIparticles therein, said particles ranging in size between 200-5000 Å;(c) the glass body is elongated under stress at a temperature above theannealing point but below that where said glass exhibits a viscosity ofabout 10⁸ poises, such that the particles are elongated to an aspectratio of at least 5:1; and (d) the elongated glass body is exposed to areducing environment at a temperature between 250° C. and about 25° C.above the annealing point of the glass to reduce at least a portion ofthe silver halide particles in the glass to silver particles which isdeposited in or on the elongated particles.

[0008] Referenced publications 1 and 2 disclose glasses employed inpolarizing glass, for example, exhibiting photochromic characteristicsand having a composition consisting essentially, expressed in terms ofweight percent on the oxide basis, of 6-20 percent R₂O (where R₂Oconsists of 0-2.5 percent Li₂O, 0-9 percent Na₂O, 0-17 percent K₂O, and0-6 percent Cs₂O), 14-23 percent B₂O₃, 5-25 percent Al₂O₃, 0-25 percentP₂O₅, 20-65 percent SiO₂, 0.004-0.02 percent CuO, 0.15-1.3 percent Ag,0.1-0.25 percent Cl, and 0.1-0.2 percent Br, the molar ratio R₂O:B₂O₃ranging between about 0.55-0.85 when the composition is essentially freefrom divalent metal oxides other than CuO, and the weight ratioAg:(Cl+Br) ranging between about 0.65-0.95.

[0009] In such types of polarizing glasses, the silver halide that isreduced in the silver halide crystal reducing step is just the outerlayer portion, with silver halide crystals being present in the glassmatrix in large quantity. When the silver halide exhibits photochromiccharacteristics, exposure to ultraviolet or visible light causesdarkening and absorption of near infrared light, compromising thepolarization characteristics of the polarizing glass, and in particular,causing a significant transmission loss.

[0010] Thus, Referenced publication 1 discloses a molar ratio of(R₂O—Al₂O₃):B₂O₃<0.25 and essentially the absence of CuO in theabove-recited composition exhibiting photochromic characteristics as acomposition rendering polarizing glass non-photochromic.

[0011] Japanese Patent No. 2628014 (Referenced publication 3) disclosesanother type of non-photochromic polarizing glass. Referencedpublication 3 points out the problem in Referenced publication 1 that,in the glass batch melt or during the heat treatment generating silverhalide crystals, silver is reduced to a metallic state and silver halidecrystals are not generated in the heat treatment the purpose of which isto generate silver halide crystals, and describes a non-photochromic,silver halide-comprising, polarizing glass composition in the form of acomposition comprising essentially no silver and a quantity of CeO₂adequate to effectively maintain the silver in the glass in an oxidizedstate. CeO₂ oxidizes silver, and is employed as an oxidizing agent forthe silver in place of CuO, which is thought to cause photochromism,thereby preventing the development of photochromic characteristics.

[0012] In such polarizing glasses, it is extremely important tostabilize the glass because of the use of a heat treatment step todeposit silver halide. However, the above-recited composition ofpolarizing glass has drawbacks in that the glass is thermally unstableand the glass loses transparency during the course of the heattreatment, that is, a haze is generated in the glass as the result ofthe deposition of crystals other than silver halide crystals. As aresult, light entering the polarizing glass is scattered andtransmission loss increases. In recent years in particular, since higherextinction ratios and lower losses have been demanded of the polarizingglass employed in optical components in the field of opticalcommunications and the like, the increase in transmission loss is amajor problem.

[0013] Further, in the manufacturing of non-photochromic polarizingglass, the CeO₂ employed in Referenced publication 3 has the sameoxidizing effect as CuO and effectively prevents the reduction of Ag, asindicated by the formulas given below. However, in methods adding anoxidizing agent such as CuO and CeO₂, the quantities added to preventreduction of Ag in the course of melting the glass must be changed basedon the melt environment and melt conditions. Further, Cu²⁺, Cu⁺, Ce⁴⁺,and Ce³⁺ ions coexist in the glass. Since the chemical equilibrium ofthese ions tends to vary with temperature, there is a risk that silverwill be reduced to a metallic state in the subsequent heat treatmentstep used to form silver halides.

[0014] 2CuO<=>Cu₂O+O 2CeO₂<=>Ce₂O₃+O

[0015] Even the further addition of CeO₂ does not fully preventphotochromism, but causes nucleation promoting the growth of deposits ofundesirable crystals other than Ag halide crystals. This is problematicin that it increases transmission loss.

[0016] The present invention, devised in light of the above-describedproblems, has for its object to provide polarizing glass with lowtransmission loss and a high extinction ratio. A further object of thepresent invention is to provide a polarizing glass permitting thereduction of silver without the deposition of metallic silver in theheat treatment step for generating a glass melt and silver halidecrystals essentially without the addition of oxidizing agents such asCuO and CeO₂.

SUMMARY OF THE INVENTION

[0017] The composition of photochromic glass is similar to that of thebase glass employed in such polarizing glasses. The present inventorsare the inventors of record of Japanese Patent Application ExaminedPublication No. Sho 56-51143 (Referenced Publication 4) disclosing thecomposition of photochromic glass comprising silver halide crystals foruse in eyeglasses.

[0018] In glass for use in eyeglass lenses, there is the technicalproblem of conforming to the standard refractive index (Nd 1.523).Japanese Patent Application Examined Publication No. Sho 56-51143describes the effectiveness, when incorporating refractive index raisingcomponents in the form of TiO₂ and ZrO₂, of keeping the quantity of TiO₂low and incorporating ZrO₂ into a composition with little Al₂O₃, therebyyielding a thermally stable glass with a low liquidous temperature at arefractive index of 1.5 and above and better photochromic performanceand chemical durability than compositions comprising large quantities ofAl₂O₃.

[0019] The extinction ratios demanded of polarizing glass in opticalcomponents have been steadily increasing in recent years (for example,40-50 dB or more at the chief wavelengths (center wavelengths of 1.31 μmand 1.55 μm) employed in the field of optical communications). In suchhigh-performance polarizing glasses, the reduction of transmission lossis an extremely important problem. Accordingly, the present inventorsemployed means such as those described in above-cited Referencedpublication 4 in the composition of the base glass of polarizing glassas well, discovering that the thermal stability of the glass wasincreased relative to the polarizing glasses disclosed in Referencedpublications 1 and 2, preventing the scattering of incident light byavoiding the loss of transparency of glass due to heat treatment; thatthe optical scattering caused by differences in refractive index withsilver halide crystals was reduced by increasing the refractive index ofthe glass; and finally, that when employed as polarizing glass, thetransmission loss of incident light could be reduced. The presentinvention was devised on that basis.

[0020] Specifically, the present invention relates to a polarizing glasscomprising geometrically anisotropic particles dispersed in an orientedmanner in at least the surface of a glass base body, characterized inthat the glass base body is denoted by the weight percentages of

[0021] 50-65 percent SiO₂,

[0022] 15-22 percent B₂O₃,

[0023] 0-4 percent Al₂O₃,

[0024] 2-8 percent ZrO₂,

[0025] 6 percent <Al₂O₃+ZrO₂<12 percent,

[0026] 6-16 percent R₂O (where R denotes at least one from among Li, Na,and K),

[0027] 0-3 percent Li₂O,

[0028] 0-9 percent Na₂O,

[0029] 4-16 percent K₂O,

[0030] Li₂O+Na₂O<K₂O,

[0031] 0-7 percent BaO and/or SrO, and

[0032] 0-3 percent TiO₂;

[0033] by comprising per 100 weight percent of essentially the abovecomposition at least 0.15-1.0 percent Ag and at least the chemicalequivalent to Ag of Cl and/or Br; and in that the geometricallyanisotropic silver particles are metallic Ag particles.

BRIEF DESCRIPTION OF THE DRAWING

[0034]FIG. 1 is a spectral transmittance curve of the glass ofEmbodiment 1 following heat treatment (but prior to reduction) and theglass of Comparative Example 2 following heat treatment (prior toreduction).

[0035]FIG. 2 is a spectral transmittance curve of the polarizing glassobtained in Embodiment 1.

[0036] The most characteristic aspects of the above-stated glasscomposition are the low quantity of Al₂O₃, the enhancement of thethermal stability of the glass by incorporation of ZrO₂, and theprevention of the deposition of undesirable metallic silver colloidsduring the heat treatments used to melt the glass and deposit silverhalides.

[0037] The above-described glass becomes photochromic if 0.002-0.03weight percent of CuO is doped, and non-photochromic glass ifessentially no CuO is doped. When the incident light is in the visiblerange, photochromic characteristics are to be avoided because absorptiondue to photochromism results in insertion losses. When the incidentlight is in the infrared range, photochromic characteristics arenormally not a problem; however, even slight absorption due tophotochromism is thought to possibly affect the infrared range and isthus desirably avoided. In either case, photochromic characteristicshave been unnecessary in polarizing glasses for optical components inrecent years, with the lack thereof being desirable.

[0038] In non-photochromic polarizing glass, based on the glass of thepresent invention, even when an oxidizing agent such as CuO is notincorporated, reduction to metallic silver in the heat treatment step tomelt the glass or generate silver halide may be fully prevented. This isbecause the base glass is highly basic. Thus, K₂O is incorporated as anessential component in the present invention. Further, more K₂O can beincorporated than Li₂O+Na₂O to keep the glass highly basic. Preferably,an RO component (an alkaline earth metal oxide), particularly BaO, isfurther incorporated to render the base glass highly basic. Since Cu²⁺ions absorb near infrared radiation, it is possible to reducetransmission loss in the near infrared range by not incorporating CuO.

[0039] The composition of the polarizing glass of the present inventionwill be described in greater detail.

[0040] The SiO₂ content is 50-65 weight percent. A content of less than50 weight percent is undesirable in that chemical durabilitydeteriorates, and a content of greater than 65 weight percent isundesirable because melting becomes difficult. The preferred SiO₂content is 55-62 weight percent.

[0041] The B₂O₃ content is 15-22 weight percent. A content of less than15 weight percent is undesirable in that silver halide particles tendnot to deposit, and a content of greater than 22 weight percent isundesirable in that the chemical durability of the glass deteriorates.The preferred B₂O₃ content range is 16-20 weight percent.

[0042] The R₂O content is 6-16 weight percent. A content of less than 6weight percent is undesirable in that melting becomes difficult, and acontent of greater than 16 weight percent is undesirable in that silverhalide particles tend not to deposit. R is at least one selected fromamong Li, Na, and K. The preferred R₂O content is 8-12 weight percent.Further, the Li₂O content is 0-3 weight percent, the Na₂O content is 0-9weight percent, and the K₂O content is 4-16 weight percent, withLi₂O+Na₂O<K₂O. Li₂O lowers the viscosity of the glass, enhancing meltingproperties. However, at greater than 3 weight percent, the glass itselftends to undergo phase separation and crystallize. Na₂O may also beincorporated, but it must be kept to not greater than 9 weight percentto keep the glass basic. K₂O is a useful component for increasing thebasicity of the glass. However, the effect is weak and the viscosity ofthe glass increases at less than 4 weight percent, and silver halideparticles tend not to deposit at greater than 16 weight percent.Further, making Li₂+Na₂O<K₂O heightens the basicity of the glass.

[0043] The Al₂O₃ content is 0-4 weight percent and the ZrO₂ content is2-8 weight percent.

[0044] When Al₂O₃ is greater than 4 percent, the thermal stability ofthe glass deteriorates in the combination with ZrO₂, and transparencytends to be lost. When the ZrO₂ content is less than 2 weight percent,chemical durability cannot be achieved in the combination with Al₂O₃ andno refractive index heightening effect is achieved. When the ZrO₂content exceeds 8 weight percent, no thermal stability is achieved withrespect to crystallization of the glass.

[0045] The total quantity of Al₂O₃ and ZrO₂ is greater than 6 weightpercent and less than 12 weight percent. When the total quantity ofAl₂O₃ and ZrO₂ is 6 weight percent or less, it is impossible to achieveglass with good chemical durability, and at 12 weight percent and above,the glass becomes unstable. Further, when ZrO₂ is incorporated insteadof reducing the quantity of Al₂O₃, since the effect of ZrO₂ onpreventing the reduction of silver is thought to be stronger than thatof Al₂O₃, the preventive effect on the reduction of silver is thought toincrease.

[0046] The Al₂O₃ content is desirably 1-3.5 weight percent, and the ZrO₂content is desirably 4-7 weight percent. The combined quantity of Al₂O₃and ZrO2 is desirably 7-10 weight percent.

[0047] Either one, or both, of BaO and SrO are desirably incorporated toincrease the basicity of the glass. A content thereof exceeding 7 weightpercent is undesirable in that silver halide particles tend not todeposit. To increase basicity and prevent the reduction of silver,holding BaO to 0.5-5 weight percent is particularly desirable.

[0048] Since TiO₂ contributes to increasing the refractive index of theglass and absorbs light of ultraviolet to short visible wavelengths thatcauses photochromism, it is incorporated with particular desirabilitywhen rendering the glass non-photochromic. In that case, a contentexceeding 3 weight percent is undesirable in that it increases thecrystallization tendency of the glass to lose transparency. Thepreferred TiO₂ content is 0.5-2 weight percent.

[0049] Ag is incorporated in a proportion of 0.15-1.0 weight percent per100 weight percent of the above-stated composition comprised essentiallyof oxides. When the Ag content is less than 0.15 weight percent, thequantity of silver halide particles deposited decreases and a highextinction ratio is precluded. A content of greater than 1.0 weightpercent is excessive, resulting in a high insertion loss, the depositionof silver halide particles in the glass during cooling followingmelting, and difficulty in controlling the size of the silver halideparticles.

[0050] Since Cl and/or Br cause deposition of silver halide particles inthe glass, a chemically equivalent quantity that is greater than the Agdissolved as Ag ions is required. However, since it tends to volatizeduring melting of the glass, the chloride or bromide of an alkali metalor alkaline earth metal is added in excess as a supplement toincorporation as AgCl and AgBr. The quantity of Cl and/or Br added inexcess differs based on the method of melting the glass and the scale,but is normally 0.3-0.6 weight percent.

[0051] Changes in the state of Ag in the process of manufacturing thepolarizing glass of the present invention will be described below.

[0052] Ag is dispersed and melted in the form of Ag⁺ ions into the baseglass body. The base glass is heat treated so that the Ag⁺, Cl⁻, and Br⁻that have been dispersed and melted as ions into the base glass diffuseinto the glass, the Ag⁺ chemically bonds with the Cl⁻ and Br⁻ to formsilver halide molecules, and these molecules then aggregate into silverhalide particles and deposit out of the glass. Subsequently, whendrawing (elongation) is conducted, the silver halide particles melt andform liquid droplets at the temperature at which the glass is drawn, andare readily elongated during elongation of the glass. Thus, elongatedgeometrically anisotropic silver halide is present in the drawn glass.Subjecting the glass following drawing to a reducing heat treatment in ahydrogen gas environment causes hydrogen atoms to diffuse into theglass, causing the surfaces of the solid-phase silver halide particlesto be reduced while still retaining their geometrical anisotropy, withsome or all of the Ag⁺ ions becoming metallic silver particles.Accordingly, the quantity of Ag element in the glass body does notchange before and after the reduction.

[0053] There is essentially no change in the composition followingreduction with regard to the other components. However, some loss of Cland Br in the form of HCl and HBr generated during the reductionreaction may occur.

[0054] The polarizing glass of the present invention comprisesgeometrically anisotropic particles that have been dispersed in anoriented manner in at least the surface layer of the glass body of theabove-stated composition, and these geometrically anisotropic particlesare metallic silver particles.

[0055] In the polarizing glass of the present invention, the layercomprising the geometrically anisotropic silver particles extends fromthe surface through part or all of the glass body. The thickness of thislayer is 20-100 μm, for example. Further, the shape of the geometricallyanisotropic particles is an ordinary slender spindle shape, the majoraxis of the geometrically anisotropic particles falls within the rangeof 300-1500 nm, for example, and the aspect ratio thereof ranges from5-60, for example.

[0056] The method of manufacturing the polarizing glass of the presentinvention will be described next.

[0057] The glass is prepared within the ranges of the above-statedcomposition and melted by known methods. Subsequently, the glass isshaped and heat-treated and silver halide particles are deposited out.The heat treatment temperatures employed in these processes are greaterthan the yield temperature of the glass and less than the temperature atwhich the silver halide crystals remelt in the glass; these processescan be conducted at a temperature of about 600-950° C.

[0058] The glass is then drawn. However, prior to drawing, the preformis desirably polished and/or etched with an acid to shape the preforminto a sheet for drawing. The predrawing polishing and/or etching isextremely effective in preventing breakage to the glass during drawing.Most preferably, both polishing and etching are performed in that order.Drawing can be conducted at temperatures at which the viscosity of theglass reaches 1×10⁶-1×10¹⁰ poise. However, to elongate the silver halideparticles without breakage the glass, drawing is preferably conducted ata temperature yielding a viscosity of greater than 2×10⁶ and not greaterthan 7×10⁷ poise. For the same reasons, the preferred stress duringdrawing is 50 kg/cm²-600 kg/cm².

[0059] The elongated glass is heat treated in a reducing environment toreduce the silver halide in the surface layer of the glass. This heattreatment can be conducted at atmospheric pressure at a temperature of350-460° C., for example, while maintaining a flow of hydrogen gas.

[0060] [Embodiments]

[0061] The present invention is further described below throughembodiments.

[0062] Table 1 presents the embodiments of the present invention. Theglass compositions, appearance of the glass following heat treatment todeposit silver halide crystals, and presence or absence of photochromismof glasses based on the related art disclosed in Japanese PatentApplication Examined Publication No. Hei 9-2628014 (Referencedpublication 3) and Japanese Patent Application Un-examined PublicationNo. Sho 56-169140 (Referenced publication 2) are given as ComparativeExamples 1 and 2. TABLE 1 Embodiments and comparative examples of thepresent invention (components given as weight percentages) Comp. Ex. 1Comp. Ex. 2 Referenced Referenced publication 3 publication 2 Embodiment1 Embodiment 2 Embodiment 3 Embodiment 4 No. 3, Table 1 No. 10, Table 1SiO₂ 59.1 57.5 61.0 59.1 56.3 58.6 B₂O₃ 18.1 20.5 17.2 18.1 18.1 18.3Al₂O₃ 2.0 3.5 — 2.0 6.2 9.51 Li₂O₃ 1.8 1.8 2.2 1.8 1.8 1.86 Na₂O — — 1.0— 4.1 2.95 K₂O 8.1 9.0 6.9 8.1 5.7 9.77 BaO 3.4 1.2 2.2 3.4 — — TiO₂ 1.5— 2.0 1.5 2.3 — ZrO₂ 5.9 6.5 7.5 5.9 5.0 — Ag 0.3 0.4 0.4 0.3 0.22 0.32CuO — — — 0.016 — 0.016 CeO₂ — — — — — — Cl 0.6 0.5 0.6 0.6 0.24 0.30 Br— 0.3 — — 0.20 0.15 Heat Treatment 760° C. 1 hr 730° C. 2 hrs 740° C. 5hrs 730° C. 8 hrs 720° C. 2 hrs 720° C. 20 min Mean particle size 110 μm95 μm 100 μm 113 μm — — Appearance Slightly opaque, Slightly opaque,Slightly opaque, Slightly opaque, Dark purple Dark purple following heattranslucent translucent translucent translucent hazy, hazy, treatmenttranslucent translucent Deposition of Absent Absent Absent AbsentPresent Present metallic silver Deposition of Absent Absent AbsentAbsent Present Absent crystals other than (rutile TiO₂ silver halidecrystals) Photochromism Absent Absent Absent Present Slight amountPresent Elongation 685° C. 675° C. 695° C. 685° C. — — temperatureElongation tension 177 Kg/cm² 200 Kg/cm² 200 Kg/cm² 177 Kg/cm² Reductionheat 440° C. 16 hrs 430° C. 8 hrs 450° C. 4 hrs 440° C. 16 hrs — —treatment Extinction ratio 1.31 μm 54 dB 56 dB 55 dB 53 dB — — 1.55 μm50 dB 50 dB 54 dB 52 dB Insertion loss 1.31 μm 0.04 dB 0.03 dB 0.03 dB0.04 dB — — 1.55 μm 0.04 dB 0.03 dB 0.03 dB 0.04 dB

[0063] In Table 1, the absence or presence of photochromism wasdetermined by irradiating, with a 500 W xenon lamp at 50 cm for 10 min,glass that had been ground to a thickness of 2 mm following heattreatment, visually observing the change in color of the glass due toirradiation, and measuring the change in transmittance at a wavelengthof 650 nm following irradiation.

[0064] The extinction ratio and insertion loss are values obtained bymeasurement of glass samples 0.2 mm in thickness having ananti-reflection coating to both surfaces.

[0065] The extinction ratio of the polarizing glass was obtained bycreating a parallel beam of light with a fiber collimator fromsemiconductor laser beams of various wavelengths, directing this beamvia a phase compensator and Grant Thomson prism perpendicularly into thepolarizing glass being measured, rotating the polarizing glass within aplane perpendicular to the optical axis, first measuring the minimumtransmitted light intensity P₁, rotating the polarizing glass 90° andmeasuring the maximum transmitted light intensity P₂, and employingEquation (1) below. The loss was obtained by measuring the lightintensity P₀ in the absence of polarizing glass and employing Equation(2) below.

Extinction ratio (dB)=−Log(P ₁ /P ₂)  (1)

Loss (dB)=−Log(P ₂ /P ₀)  (2)

[0066] When this polarizing glass is employed in an opticalcommunication optical isolator, it is necessary to minimize Fresnelreflection on the surface. Thus, a reflection-reducing film is normallyformed on the polarizing glass. An SiO₂/TiO₂/SiO₂ three-layeredreflection-reducing film the thickness of which was designed to minimizereflectance at each of the wavelengths employed was formed on modifiedglass of the present embodiment. The reflectance was 0.1-0.2 percent onboth surfaces.

[0067] Embodiment 1

[0068] Glass having the composition of Embodiment 1 in Table 1 wasmelted at about 1,450° C. in a five-liter platinum crucible, poured intoa mold, and gradually cooled at 530° C. to produce a mother glass block.Commonly employed optical glass starting materials in the form of theoxides SiO₂, TiO₂, and ZrO₂ as well as compounds such as H₃BO₃, Al(OH)₃,Li₂CO₃, Na₂CO₃, NaNO₃, K₂CO₃, KNO₃, BaCO₃, and Ba(NO₃)₂, and silver andchlorine starting materials such as Ag₂O, AgCl, NaCl, KCl, and BaCl₂,can be employed as glass starting materials.

[0069] The glass block was then cut to about 70×230×10 mm, placed in amold of refractory material, and heat treated in an electric furnace for1 hr at 760° C., a temperature 60° C. higher than the softening point ofthe glass, yielding glass in which silver chloride crystal particles haddeposited. Observation by transmission electron microscopy revealed theaverage particle size of the silver chloride that had deposited out tobe 110 nm. Glass that was colorless and transparent prior to the heattreatment developed white haze and become translucent due to thedeposition of silver chloride crystals following the heat treatment.However, as indicated by plot (1) of the spectral transmittance curve (2mm in thickness) of FIG. 1, absolutely no absorption or coloration dueto the deposition of reduced silver colloids was observed. Further, onlydiffraction pattern of silver chloride crystals were observed in theX-ray diffraction pattern of glass with an average particle size of 133nm that had been subjected to an excessive heat treatment for 8 hr at750° C., confirming that no crystals other than silver chloride haddeposited out.

[0070] This heat-treated glass slab was processed into the form of asheet measuring about 60×220×2 mm and subjected to polishing. The sheetwas then etched for 5 min with a mixed solution of hydrofluoric acid andsulfuric acid to remove scratches from the surface, yieldingsheet-shaped preforms for elongation.

[0071] These preforms were loaded into the feeder of an elongationdevice equipped with both a preform feeding mechanism and pullingmechanism in the top and bottom portions of an electric furnace, a 100 gweight was attached, and heating was conducted once the front edge ofthe preform had been positioned slightly below the center of the furnacewhere the highest temperatures are reached. The temperature wasincreased to 685° C., corresponding to a glass viscosity of 8×10⁷ poise,and maintained at that level for about 30 min, at which time the frontedge portion of the preform softened. The neck dropped down, droppingout of the opening at the bottom of the furnace. This was gripped bypulling rollers equipped with a motor drive mechanism and pulling wasbegun. Elongation was conducted at a preform feed rate of 25 mm/min anda pull rate of 0.4 m/min, continuously yielding about 1.5 m ofribbon-shaped glass measuring about 15×0.47 mm in cross section. Thepull load during elongation as measured with a load cell was 12.5 Kg,and the elongation tension obtained by division by the cross-sectionsurface area of the ribbon was 177 Kg/cm².

[0072] The elongated glass ribbon was cut into a short tabular shapeabout 65 mm in length, both surfaces were ground to a thickness of 0.2mm, and under a hydrogen gas flow, a reducing heat treatment wasconducted for 16 hr at 430° C. The transmittance curve of the glassobtained is shown in FIG. 2. Plot (1) is the case where polarizing lightwas introduced in parallel to the polarizing transmission axis and plot(2) is the case where polarizing light was introduced perpendicular tothe transmission axis, with the absorption peak wavelength of plot (2)being about 1.27 μm. Further, observation by transmission electronmicroscopy revealed the mean aspect ratio of the elongated particles tobe about 14. Reflection-reducing films for 1.31 μm and 1.55 μm wereformed on both surfaces of the glass and the extinction ratio andinsertion loss were measured at each of the wavelengths, yielding valuesof 54 dB and 0.04 dB, and 50 dB and 0.04 dB, respectively.

[0073] Glass that was manufactured with an elongation load that had beenincreased to 14 Kg (a tension of 197 Kg/cm²) but otherwise under thesame conditions as those set forth above had a peak absorbancewavelength of 1.48 μm, an aspect ratio of 17, an extinction ratio of 52dB and an insertion loss of 0.04 dB at a wavelength of 1.31 μm, and anextinction ratio of 56 dB and an insertion loss of 0.04 dB at awavelength of 1.55 μm.

[0074] Embodiments 2-4

[0075] Embodiments 2 and 3 had different compositions than Embodiment 1.In Embodiment 4, 0.016 weight percent of CuO was added to Embodiment 1.Using the same procedure as in Embodiment 1, polarizing glasses weremanufactured from these glasses under the same conditions as inEmbodiment 1. All the glasses had good polarizing characteristics in theform of extinction ratios of not less than 50 dB and insertion losses ofnot greater than 0.05 dB.

[0076] For all of these glasses, absolutely no reduced metallic silveror deposited crystals other than silver halide crystals were observedeither in the melt or in the glass following the heat treatment todeposit silver halide crystal particles.

[0077] Further, although Embodiment 4, comprising a trace quantity ofCuO, was sensitive to ultraviolet radiation, darkened, and exhibitedphotochromism, Embodiments 2 and 3 had no optical sensitivity and werenon-photochromic in the same manner as Embodiment 1.

[0078] Based on these embodiments, so long as no CuO is comprised in theglass of the present invention, no silver ions are reduced to metallicsilver, depositing out and lowering transmittance (causing coloration),and no crystals other than silver halide crystals deposit out. That is,the factor causing the deterioration of polarization characteristics inpolarizing glass has been eliminated.

Comparative Examples 1 and 2

[0079] Glasses of the composition of Embodiment No. 3 of Table 1 inJapanese Patent Application Examined Publication No. Hei 9-2628014(Referenced publication 3) comprising large amounts of Al₂O₃ and ZrO₂but no BaO (Comparative Example 1), and of the composition of EmbodimentNo. 10 of Table 1 in Japanese Patent Un-examined Publication No. Sho56-169140 (Referenced publication 2) comprising no BaO (ComparativeExample 2) were prepared. Photochromism and the appearance of theglasses following heat treatment to cause deposition of silver halidecrystals were compared with those of the glasses of the presentapplication.

[0080] As set forth in the Specification section, reduced metallicsilver and rutile crystals deposited out in addition to silver halidecrystals in the glass of Embodiment No.3 of Japanese Patent PublicationNo. Hei 9-2628014 due to a heat treatment for 2 hr at 720° C. As aresult, the glass exhibited dark purple haze and slight photochromism.The glass of Referenced publication 3 comprised essentially no copper,was non-photochromic, and was characterized by comprising an amount ofCeO₂ adequate to effectively maintain the silver in an oxide statewithin the glass. This can be interpreted to mean that in a compositioncomprising more Al₂O₃ than in the present invention, no BaO, and littleK₂O, the content of CeO₂ is essential to maintaining the silver in anoxide state in the glass.

[0081] Embodiment No. 10 of Referenced publication 2 is a photochromicglass comprising CuO. However, despite the incorporation of CuO, theglass exhibited a dark purple coloration similar to that of ComparativeExample 1 when heat treated for 20 min at 720° C. and a loweredtransmittance over a broad wavelength range as shown in plot (2) ofFIG. 1. This indicates that silver tends to be reduced in glassescomprising large quantities of Al₂O₃.

[0082] In the above-described embodiments, polarization characteristicsare described for the wavelength range of 1.3-1.6 μm that is principallyemployed in optical communication. However, since the factor causingdeterioration of polarization characteristics has been eliminated in thepresent invention, it is clearly effective at other wavelengths. Forexample, it can be suitably applied to the polarizers employed inoptical isolators for 0.98 μm light source for excitation of an opticalamplifier.

[0083] Based on the present invention, polarizing glass can bemanufactured in which metallic silver and crystals other than silverhalide causing deterioration of polarizing characteristics such as theextinction ratio and insertion loss do not deposit out, and having goodpolarizing characteristics (for example, an extinction ratio of notgreater than 50 dB and an insertion loss of not greater than 0.5 dB at acenter wavelength of 1.31 μm and/or 1.55 μm) due to beingnon-photochromic. Further, since it is possible to prevent thedeposition of metallic silver without employing CeO₂, with its unstableeffect as a silver oxidizing agent and promotion of the growth ofdeposits of crystals other than silver halide that function asnucleating agents, high-performance polarizing glass can be stably andreliably manufactured.

[0084] The present disclosure relates to the subject matter contained inJapanese Patent Application No. 2001-289444 filed on Sep. 21, 2001,which is expressly incorporated herein by reference in its entirety.

What is claimed is:
 1. A polarizing glass comprising geometricallyanisotropic particles dispersed in an oriented manner in at least thesurface of a glass base body, wherein the glass base body is denoted bythe weight percentages of 50-65 percent SiO₂, 15-22 percent B₂O₃, 0-4percent Al₂O₃, 2-8 percent ZrO₂, 6 percent <Al₂O₃+ZrO₂<12 percent, 6-16percent R₂O (where R denotes at least one from among Li, Na, and K), 0-3percent Li₂O, 0-9 percent Na₂O, 4-16 percent K₂O, Li₂O+Na₂O<K₂O, 0-7percent BaO and/or SrO, and 0-3 percent TiO₂; comprises per 100 weightpercent of essentially the above composition at least 0.15-1.0 percentAg and at least the chemical equivalent to Ag of Cl and/or Br; and thegeometrically anisotropic silver particles are metallic Ag particles. 2.The polarizing glass according to claim 1 wherein the glass comprises0.5-5 weight percent BaO.
 3. The polarizing glass according to claim 1wherein the glass comprises 0.002-0.03 weight percent CuO.
 4. Thepolarizing glass according to claim 1 wherein the glass substantiallydoes not comprise CuO and o substantially does not exhibit photochromiccharacteristics.
 5. The polarizing glass according to claim 1 whereinthe glass comprises 1-3.5 weight percent Al₂O₃.
 6. The polarizing glassaccording to claim 1 wherein the glass comprises 4-7 weight percentZrO₂.
 7. The polarizing glass according to claim 1 wherein the glasscomprises 7-10 weight percent Al₂O₃ and ZrO₂.
 8. A process forpreparation of a polarizing glass comprising steps of; heat treating ashaped glass having the composition denoted by the weight percentages of50-65 percent SiO₂, 15-22 percent B₂O₃, 0-4 percent Al₂O₃, 2-8 percentZrO₂, 6 percent <Al₂O₃+ZrO₂<12 percent, 6-16 percent R₂O (where Rdenotes at least one from among Li, Na, and K), 0-3 percent Li₂O, 0-9percent Na₂O, 4-16 percent K₂O, Li₂O+Na₂O<K₂O, 0-7 percent BaO and/orSrO, and 0-3 percent TiO₂; comprising per 100 weight percent ofessentially the above composition at least 0.15-1.0 percent Ag and atleast the chemical equivalent to Ag of Cl and/or Br to deposit outsilver halide particles; drawing the glass to elongate the silver halideparticles in the glass; and reducing at least part of the elongatedsilver halide particles in the glass to form geometrically anisotropicsilver particles.
 9. The process for preparation of claim 8 wherein theshaped glass is polished and/or etched.